1. Field of the Invention
The present invention relates to a lithium rechargeable battery that includes a current interruption means comprising a bimetal that is actuated by the inner pressure or heat that is generated from within the rechargeable battery in the case of a malfunction to interrupt the flow of current for improved stability of the battery.
2. Description of the Background
Lithium rechargeable batteries are classified into cylindrical lithium rechargeable batteries and rectangular-box type lithium rechargeable batteries based on their external appearance. The cylindrical lithium rechargeable batteries have a cap assembly that interrupts the current within the rechargeable batteries when the internal pressure of the batteries rises above a predetermined level and there is a danger of explosion due to overcharge or malfunction. In addition, the current may be interrupted to suppress any further reaction for improved safety of the lithium rechargeable batteries.
The structure of a cap assembly of such cylindrical lithium rechargeable batteries is disclosed in Korean Patent No. 10-0357950. The cylindrical lithium rechargeable battery includes a cylindrical can and a cap assembly that is hermetically coupled to the top opening of the can with an insulation gasket interposed between them. The cylindrical can contains an electrode assembly that is formed by winding a positive electrode plate and a negative electrode plate into a jelly roll with a separator inserted between them, and an electrolyte.
The cap assembly may include a safety vent, a current interruption means, a secondary protective device, and a terminal cap (also referred to as a cap-up). The safety vent may have a plate shape and is positioned below the cap assembly. The safety vent has a protrusion that is formed at the center thereof while protruding downward such that the protrusion is deformed upward by the pressure that is generated from within the rechargeable battery. An electrode tab is drawn from the positive electrode plate and the negative electrode plate of the electrode assembly. For example, the positive electrode tab that is drawn from the positive electrode plate is welded to the lower surface of the safety vent in a predetermined position to couple the safety vent to the positive electrode plate. The negative electrode plate may be directly coupled to the can or coupled via a tab.
The current interruption means is positioned on top of the safety vent and allows current to flow to the secondary protective device via the safety vent. If the battery malfunctions or its internal pressure rises above a predetermined level, the protrusion of the safety vent deforms upward and is destroyed by the current interruption means. The flow of current is then interrupted.
Conventional current interruption means may include a circular outer ring, a bar that extends through the center of the outer ring, an insulated print board that is positioned at the center of the bar and has a via hole formed thereon, and conductive films that are positioned on the upper and lower portions of the insulated print board. A side of an upper conductive film is connected to the outer ring of the insulated print board and the other side thereof is not connected thereto. In contrast, a end of a lower conductive film is not connected to the outer ring and the other end thereof is also connected thereto. The via hole has a conductive layer formed thereon which is made of a conductive metal such as copper to couple the upper and lower conductive films to each other.
The bar has fracture portions that are formed on both lateral surfaces of the portion on which the via hole is formed so that the bar can easily be fractured.
The current interruption means is positioned to contact the protrusion of the safety vent and extends through the via hole. If the protrusion of the safety vent protrudes upward, the resulting force fractures the bar of the current interruption means about the via hole. The electrical connection between the conductive layer within the via hole and the upper or lower conductive film is then broken. As such, the current interruption means are adapted to interrupt the current between the safety vent and the secondary protective device.
However, conventional current interruption means are problematic because although the conductive layer that is formed within the via hole is supposed to be completely separated from the upper or lower conductive films when the bar is fractured by the safety vent, the conductive layer sometimes fails to be completely separated because it is made of a ductile metal such copper. Specifically, the conductive layer may stretch due to ductility while still being coupled with the upper and lower conductive films even when the bar has been fractured, and fails to separate the upper and lower conductive films from each other. The rechargeable battery then continues to undergo chemical reactions and may explode.
Furthermore, conventional current interruption means are actuated only when the protrusion of the safety vent is inverted by the internal pressure of the rechargeable battery. Thus, it may fail to interrupt the flow of current in the rechargeable battery unless equipped with a separate secondary protective device that reacts to temperature changes.